Many industrial operations require the use of high purity, particle-free fluids. In particular, the manufacture of semiconductor-based integrated circuits requires the regular use of very aggressive fluids such as strong mineral acids, corrosive materials such as HF, concentrated salt solutions or combinations of such materials.
Many of these fluids, such as HF/NH.sub.4 F buffers which are used to etch patterns in silicon oxide layers deposited on high purity silicon wafers, have high surface tensions on the order of 85 dynes/cm or even higher, i.e, substantially higher than that of water which has a surface tension of 72 dynes/cm. In order to produce a particle-free fluid, these materials must be filtered. To be useful for filtering these aggressive fluids, the filter medium should be inert to the fluids, offer little resistance to flow of the fluids through the filter, and be fully wettable by the fluids.
A material commonly used in equipment which handles HF/NH.sub.4 F mixtures is polyvinylidene difluoride ("PVDF"). PVDF is inert to such mixtures, being neither weakened by them nor introducing any contaminants into them. PVDF filter media, particularly microporous membranes, can be prepared to exhibit high efficiency for particle removal. However, PVDF has a low critical surface energy and conventional, hydrophobic, microporous PVDF membranes will not wet with fluids having surface tensions higher than about 32 dynes/cm. To spontaneously wet a PVDF structure, the surface tension of the liquid must be no greater than the critical surface energy of the structure.
A commercially available, microporous PVDF membrane sold under the Durapore trademark (Millipore, Inc.) is available in a hydrophilic version which wets spontaneously with water, i.e., it has a critical surface energy of at least 72 dynes/cm, the surface tension of water. However, this membrane will not spontaneously wet with typical HF/NH.sub.4 F mixtures having surface tensions of about 85 dynes/cm. Indeed, this commercial filter membrane will not spontaneously wet with fluids having surface tensions greater than about 75 dynes/cm indicating a critical surface energy no higher than about 75 dynes/cm. Other commercial filter materials which are inert to HF/NH.sub.4 F mixtures, such as polypropylene webs and microporous polysulfone and polycarbonate membranes, are also incapable of being spontaneously wetted with commonly used fluids having high surface tensions.
Lack of spontaneous wetting of the filter membrane by the fluid to be filtered requires that the membrane be prewetted with a low surface tension fluid. This prewetting fluid must then be replaced with the fluid to be filtered before the filter can be placed into service. Typical prewetting procedures comprise (1) prewetting the filter with a low surface tension alcohol, (2) flushing the alcohol out with high purity water, and (3) flushing the water out with the fluid to be filtered. This procedure is time-consuming and each operation provides an opportunity for accidental contamination of the filter or fluid to be filtered. Spontaneous wetting with the high-surface tension fluid to be filtered is, therefore, a very desirable characteristic of a filter medium used in a filter for applications such as that described above.